Electronic waste such as, for example, waste printed circuit boards (PCBs) and waste connections from photovoltaic cells, may contains large amount of valuable metals. In view of environmental concerns, and the increasing cost of such metals, there is a need for more effective methods of recovering the metals from such waste.
Compositions designed to strip tin and/or tin-lead coatings from copper surfaces are known in the art. Most of these compositions are specifically designed to recover solder from printed circuit boards.
Peroxide-based stripping solutions are known (see, for example, U.S. Pat. Nos. 3,926,669, 4,673,521 and 5,223,087). While these solutions are effective at stripping tin and tin-lead alloy from a copper surface, the reaction is very exothermic and often requires the use of cooling coils to avoid attack of the copper surface itself as the stripping solution heats up. Such solutions are also prone to out-gassing.
Stripping solutions containing nitric acid and ferric salt are also effective at tin-stripping (see, for example, U.S. Pat. No. 4,713,144). However, the rate of reaction is typically not controllable, leading to sludge formation and copper etching. These stripping solutions are also very exothermic and, since they comprise hazardous materials, present handling and disposal problems.
Organic and inorganic sulfonic acid-based strippers are capable of stripping tin in a safe, non-exothermic manner (see, for example, U.S. Pat. Nos. 4,957,653, 4,944,851 and 4,921,571). However, such stripping solutions are expensive and are prone to result in re-deposition of tin onto the copper from the stripping bath, which can be difficult to remove without excessive attack on the copper. They are also plagued by sludge formation.
All of the above stripping solutions require an additional step to recover metal from the stripping solution, which increases the overall cost of the process. There is also uncertainty regarding whether such stripping solutions work effectively on different tin-containing alloys such as, for example, tin-silver and tin-bismuth alloys. This is of particular concern when the stripping solutions are to be used on photovoltaic scrap such as, for example, interconnection ribbons and busbars, which typically contain such alloys.